An unfortunate result of technological advances in image capture and reproduction is illegal copying and distribution of image content, in violation of copyright. One solution for counteracting illegal copying activity is the use of image watermarking as a forensic tool. Sophisticated watermarking techniques enable identifying information to be encoded within an image. A digital watermark can be embedded in the image beneath the threshold of visibility to a viewer, yet be detectable under image scanning and analysis. Examples include: U.S. Pat. No. 6,496,818 (Yoda) which discloses embedding a pattern in a color print and adjusting cyan, magenta, yellow, black (CMYK) values such that the embedded data matches the color of the surround when viewed under a standard illuminant; commonly-assigned U.S. Pat. No. 5,752,152 (Gasper et al.) discloses a pattern of microdots, less than 300 μm in diameter, for marking a photographic print that is subject to copyright.
Illegal copying is a particular concern to motion picture studios and distributors, and represents a significant loss of revenue. Watermarking of motion picture images would enable the source of an illegal copy to be tracked and would thus provide a deterrent to this activity.
While a number of different approaches have been attempted for watermark application to motion pictures, there is considerable room for improvement. For photosensitive media in general, it is known that a watermark encoding can be added to the image frame at the same time that image content is printed. However, it is also possible to expose a watermark at other times during processing of the photosensitive medium. For example, as is disclosed in U.S. Patent Application No. 2003/0012569 (Lowe et al.), a latent watermark image can be exposed onto the “raw” photosensitive medium itself, at the time of manufacture. Then, when the medium is exposed with image content, the image frame is effectively overlaid onto the watermark pattern. Such a method is also disclosed in U.S. Pat. No. 6,438,231 (Rhoads). U.S. Pat. No. 6,438,231 discloses this type of pre-exposure of the watermark onto the film emulsion within the frame area of negative film, for example.
It can be appreciated that watermark pre-exposure would have advantages for marking motion picture film at the time of manufacture or prior to exposure with image content. A length of motion picture film could be pre-exposed with unique identifying information, encoded in latent fashion, that could be used for forensic tracking of an illegal copy made from this same length of film. However, prior art watermarking techniques proposed for photosensitive media in general fall short of what is needed for motion picture watermarking. In particular, prior art techniques are not well-adapted for applying a watermark pattern during high-speed film manufacture. Problems that make it difficult or impractical to use conventional watermark application techniques for pre-exposure of film in manufacture relate to both throughput requirements and image quality. Among the problems with watermark application in high-speed manufacturing environments are the following:                (a) Energy requirements. Exposure is a product of the intensity of applied radiant energy and time. With film moving at high speeds, very little time can be allotted for exposure of a watermark; consequently, the intensity of the exposing light source used must be very high. Providing light having the intensities needed to expose small areas at film movement speeds, however, presents a formidable technical challenge. High overall energy requirements add cost and complexity to the job of applying watermark exposure. Heat dissipation can also become a problem.        (b) Exposure control. Modulation of light amplitude at high levels, with sufficiently accurate control over relative intensity levels, presents challenging technical problems, making it difficult to provide watermark pixels at varying densities. At the high light energy levels needed for high-speed watermark application, for example, a 10% error in output intensity can cause excessive noise and render watermark modulation unusable. The expense of obtaining precision, high-intensity light components or of using extensive feedback controls to counter this problem could be prohibitive.        (c) Imaging artifacts due to pixel-to-pixel exposure energy level imbalances between adjacent exposure sources. Forming a watermark exposure using digital imaging typically requires an array of exposure sources, from pixel forming devices such as LED arrays, for example. Adjacent pixel exposure sources must be closely matched for energy output; otherwise, linear artifacts such as banding can occur.        (d) Imaging artifacts due to pixel placement errors. Pixel spatial placement can be imperfect, causing recurring gaps or overlaps between adjacent pixels that could cause undesirable image artifacts.        (e) Imaging artifacts due to pixel shape anomalies. In addition to differences in relative placement, adjacent pixels may also vary in size. This type of condition may result in banding or other perceptible effects on the watermark image that are not masked by image content. Imaging artifacts may be enhanced by the contrast of the photosensitive medium.        
Thus, it can be appreciated that techniques that reduce energy requirements and minimize the impact of pixellization artifacts would be useful for digital watermark application in a high-speed film manufacturing environment.
It can be appreciated that watermark exposure, including pre-exposure for example, would have advantages for marking motion picture film at the time of manufacture or prior to exposure with image content. A length of motion picture film could be pre-exposed with unique identifying information, encoded in latent fashion, that could be used for forensic tracking of an illegal copy made from this same length of film.
In addition to watermarks, other types of latent images can be exposed onto a photosensitive medium, such as at the time of manufacture or at the time of filming. With this in mind, it can be appreciated that methods adopted for watermark application may also be well suited for forming other types of latent images, including time stamps, batch identifiers, and other types of indicia, or discriminating marks, that would be useful to the manufacturer, processor, or end user of film and other sensitized media. These latent images are typically within the image area of the sensitized media; however, latent images could alternately be formed along borders or edges of the media, wholly or partially outside the image area.
Given these considerations, it can be seen that improvements to conventional approaches for watermark application would be advantageous, particularly where there is a need to apply a latent watermark pattern during film manufacture and with other high-speed film handling systems.